Fast Curing Multifunctional Tissue Adhesives of Sericin-Based Polyurethane-Acrylates for Sternal Closure

The use of wire cerclage after sternal closure is the standard method because of its rigidity and strength. Despite this, they have many disadvantages such as tissue trauma, operator-induced failures, and the risk of infection. To avoid complications during sternotomy and promote tissue regeneration, tissue adhesives should be used in post-surgical treatment. Here, we report a highly biocompatible, biomimetic, biodegradable, antibacterial, and UV-curable polyurethane-acrylate (PU-A) tissue adhesive for sternal closure as a supportive to wire cerclage. In the study, PU-As were synthesized with variable biocompatible monomers, such as silk sericin, polyethylene glycol, dopamine, and an aliphatic isocyanate 4,4′-methylenebis(cyclohexyl isocyanate). The highest adhesion strength was found to be 4322 kPa, and the ex vivo compressive test result was determined as 715 kPa. The adhesive was determined to be highly biocompatible (on L-929 cells), biodegradable, and antibacterial (on Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus bacteria). Finally, after opening the sternum of rats, the adhesive was applied to bond the bones and cured with UV for 5 min. According to the results, there was no visible inflammation in the adhesive groups, while some animals had high inflammation in the cyanoacrylate and wire cerclage groups. These results indicate that the adhesive may be suitable for sternal fixation by preventing the disadvantages of the steel wires and promoting tissue healing.

S-2 Figure S1. Crosslink frequency of PEG200, PEG400 and PEG600-based polyurethanes Structural and Thermal Characterization FTIR spectra of sericin-based adhesives containing 20% hydroxyl and PEG200 units (HMDI-SER-P200-20) were given in Fig. S2. In these spectra, all vibration peaks arising from the polyurethane structure were seen as well as the amide vibrations arising from the sericin. The C=O intense tensile vibration at 1711 cm -1 , the N-H tensile vibration at 3300 and 1458 cm -1 of the polyurethane structure and the aliphatic C-H tensile vibration at 2900-2800 cm -1 of the PEG structure were observed. Furthermore, C-O-C tensile vibrations caused by PEG and cyclodextrin structures were clearly seen at 1274 and 726 cm -1 . These peaks proved that the desired polyurethane structure was obtained clearly.
According to the FTIR results, it can be said that acrylate groups were attached to the structure by UV-curing. After curing, increased methylene peaks and C-C tensile vibrations seen at 1616 cm -1 proved the polyurethane-acrylate structure [1][2][3] . The adhesive, which is in liquid form before curing, had the appearance of a solid, transparent S-3 and colorless polymeric film after curing. This change proved that the crosslinking efficiency was high.
FTIR spectra of sericin-based adhesives containing PEG200 and 30% free hydroxyl (HMDI-SER-P200-30) were shown in Fig. S3. In the spectrum, -OH peak at 3439 cm -1 , -CH at 2884 cm -1 and -COO at 1720 cm -1 were observed. Also, NHCOO strain at 1694 cm -1 , -NH tension at 3319 cm -1 , and N-H bending at 1537 cm -1 were seen. These structures substantially proved that the desired polyurethane structure was formed. In the red spectrum to which the acrylate units were bound on the related polyurethane structure, it can be seen that the peak at 2265 cm -1 causing from free isocyanate groups of 2isocyanatoethyl methacrylate disappeared. It proved that the acrylate group was included in the polymer structure. Also, C-H off-plane bending at 675 cm -1 originating from C=C double bond can be interpreted for the presence of acrylate structures. In the last spectrum, after the curing, this peak received a wider and band character. The expansion of the aliphatic C-H tensile vibration (2830-2950 cm -1 ) proved that curing was achieved. [1][2][3] In Fig. S4, the spectra of PEG200, sericin and 40% free hydroxyl containing polymers (HMDI-SER-P200-40) were shown. In this spectrum, basic polyurethane peaks were seen, similar to others. These are C=O (1720 cm -1 ), N-H (1582 cm -1 ) tensile vibration, C-N tensile vibration (1440 cm -1 ), C-O-C tensile vibration (1240 cm -1 ) peaks originating from the classical urethane structure. In addition, the absence of free isocyanate peaks in all spectra and the formation of these peaks proved that the urethane bond was established in the polymer. Another component was cyclodextrin groups in the structure. Due to these cyclodextrin groups, we can clearly see C-H tensile vibrations between 1000-1092 cm - Tg values of polymers were seen as 16.21, 15.61 and 16.47 °C for HMDI-SER-P200-20, HMDI-SER-P200-30 and HMDI-SER-P200-40, respectively (Fig. S5). These values increased after curing to 20.91, 17.72 and 29.08 °C, respectively. This is due to the increased rigidity with increasing crosslink ratio after curing. 4 This interpretation was seen in accordance with the DTA thermograms shown in Fig. S6. DTA thermograms showed that the structural stability of the polymer increased as the structural crosslinking increased. Decomposition start temperatures were moved to a higher value after curing. When the PEG structure was changed from PEG200 to PEG600 in the polyurethane, polyurethaneacrylate and cured structures, a higher flexibility was obtained. At the same time, as a disadvantage, the applicability of the polymer and its solubility has decreased. The spectra were given in Figure S10, Figure S11 and Figure S12. In the spectra given in Figure S10, Figure S11 and Figure   The changes in the general thermal properties of the PEG400-sericin polymers were examined by DTA, TGA and DSC analyzes. In Figure S13, DSC thermograms of the polymers containing PEG400-sericin were given. Tg values of PEG400-sericin structures were observed around 17 °C in these thermograms. This value indicated that the polymer was injectable and applicable in the body. In addition, this value increased with curing. OH groups originating from cyclodextrin were reduced and were crosslinked with PEG and diisocyanates. In addition, urethane groups also increased the crosslinking points. As a result of these cross-links, it becomes difficult for the polyurethane chains to approach each other, so a decrease in polymer resistances occurs. Due to this effect, Tg temperatures of polymers increased. DTA thermograms given in Figure S14 gave two exotherms to verify this finding. In these thermograms, an exotherm was seen due to the degradation of the polymeric binding units. Structural and thermal stability increased significantly with curing. TGA thermograms also supported these results. In TGA thermograms, first of all, 2% moisture removal was seen around 100-110 ° C. Later, a mass loss

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was observed around 250-350 °C due to the degradation of the protein and PEG groups in the polyurethane structure. Finally, degradation of cyclodextrin groups and mass losses of thermoxidative degradation were observed in Figure S15, Figure S16, Figure S17 and Figure S18.
In addition, as the structure was cured, the soft segment degradation and degradation initial temperature increased. FTIR spectra of sericin-based adhesives carrying PEG600 units were given in Figure S19, Figure S20 and Figure S21. In these spectra, the C=O tensile vibration originating from the polyurethane structure was seen at 1708 cm -1 . Also, the N-H stress vibration at 3220-3400 cm -1 was seen as a medium intensity peak. The N-O peak on the isocyanate bond was observed around 1530 cm -1 . These peaks showed that the desired urethane bond was obtained.  Figure S23 support these results. There are three main mass losses of polymers in the TGA thermogram in Figure S24, Figure S25 and Figure   S26. The first loss of mass is caused by the removal of the polymer moisture structure at low temperature. The second mass loss is due to the breakage of groups such as CH2-CH2-O-CH2 on the polymer chain. The final mass loss is thermoxidative decomposition. These values are carried to

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higher temperatures by curing the acrylate groups. The rate of this increase was seen to be more than other PEG structures.  Figure S27 proves that the desired polyurethane structure has been achieved.
Acrylate groups are bonded to polyurethane in order to provide UV-curable property. Acrylate groups were examined by NMR spectrum. In Figure S27B, 3 protons with different chemical environment are seen on acrylate groups. These protons are seen at 5.61 ppm and 6.24 ppm(a1), 6.18(a2) ppm and 1.85(a3) ppm. Especially the a3 peak, which has a sharp singlet appearance, is due to the -NH proton in acrylate structure and proves that the acrylate group is included in the structure. The hard and solid polymeric structure formed after curing is another proof of this. In the NMR spectrum in Figure S27B, the polyurethane structure peaks are given. These are seen in the spectrum as g, h, i and j peaks from PEG, 1-9 peaks from cyclodextrin, a, b, c, d, e peaks from isocyanate and p1, p2, p3, p4 peaks from sericin group in the structure. In addition, f peak at 7.20 ppm proved that the urethane bond was formed. As a result, both the pre-polymer structure and the acrylate-bonded polyurethane structure are confirmed in Figure S27. Within the study, SEM analyzes were carried out to determine the morphological properties of PU-A formulations (Fig. S29). In these analyzes, UV curing was performed after applying the polymer solution to a glass surface. The cured polymer was removed from the surfaces and SEM analyzes were carried out with these films. SEM images of flat and fractal surfaces from polymers were obtained 1000x and 5000x magnification (Fig.   S29). According to the results, polyurethane formed a homogeneous film after curing (PU-A). The detailed analysis of polymer surfaces was also examined with AFM technique (Fig.   S30). According to the results, it is seen that the surface roughness is below 50 nm.

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Therefore, the polymer structure spreads very well on the applied surfaces and creates a very thin film.   S-26